In general, in the production processes of high-density semiconductor integrated circuits such as LSI, color filters for CCD (charge-coupled device) and LCD (liquid crystal display device), magnetic heads, etc., microfabrication utilizing the photolithographic technique using photomasks is performed.
In this microfabrication, a photomask, in which a light-shielding film consisting of a metal thin film such as a chromium film is generally formed on a light transmissive substrate made of quartz glass, aluminosilicate glass or the like by means of sputtering, vacuum deposition or the like to provide a photomask blank, wherein the light-shielding film is formed to have a predetermined pattern, is used.
A photomask is produced using such a photomask blank with the following processes: an exposure process in which a desired pattern exposure is applied to a resist film formed on the photomask blank; a development process in which, after the desired pattern exposure is applied to the resist film formed on the photomask blank, a developing solution is supplied thereto to dissolve portions of the resist film soluble in the developing solution, thereby forming a resist pattern; a etching process in which, using the obtained resist pattern as a mask, portions in which a light-shielding film is exposed with the resist pattern not formed are removed by etching, such as, wet etching using an etching solution consisting of a mixed aqueous solution of ceric ammonium nitrate and perchloric acid, and dry etching using chlorine gas, thereby forming a predetermined mask pattern on a light transmissive substrate; and a stripping/removing process in which the remaining resist pattern is stripped and removed.
During patterning of the light-shielding film in the etching process, the resist pattern formed on the light-shielding film must remain with a sufficient film thickness. However, when the resist film thickness is increased, the aspect ratio is increased, and this causes the problem of pattern collapsing, etc., particularly in the case where a fine pattern is to be formed. Therefore, in order to miniaturize a mask pattern formed on a photomask, it is required to decrease the thickness of a resist film formed on a photomask blank.
Regarding this point, Japanese Laid-Open Patent Publication No. 2007-33470 (Patent Document 1) discloses a photomask blank comprising a light-shielding film having a thickness of 100 nm or less, wherein the film has a structure in which the percentage of the film thickness of a chromium-based compound having a high etching rate is 70% or more to enable reduction in etching time, thereby realizing miniaturization of the resist. Specifically, Patent Document 1 discloses a photomask blank in which a semitransparent film, a CrON film, a Cr film and a CrON film are laminated on a light transmissive substrate, wherein the percentage of the thickness of the CrON film is 70% or more.
However, regarding the above-described CrON film, it is just that the optical density per unit film thickness at a wavelength of 450 nm is set, and regarding a wavelength of exposure light equal to or less than wave length of an ArF excimer laser light, no optimization has been made. In particular, in the case of hyper-NA lithography, the angle of light incidence relative to a photomask becomes shallower, and this causes the problem that a miniaturized mask pattern itself shades a transfer image (shadowing). When a light-shielding film is thick, reduction of the amount of light (deterioration of contrast) due to shadowing is highly influential. In addition, the cross-section shape is prone to vary, and this, together with shadowing, causes reduction of transfer accuracy of CD (Critical Dimension).    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2007-33470